Metallurgy and Processing of High-Integrity Light Metal Pressure Castings

Von Helmut Kaufmann

Very sophisticated pressure casting equipment, such as High Pressure Die Casting (HPDC) machines with hydraulic shot control, Vertical Squeeze Casting machines and various Semi-solid Casting machines are used in modern foundries today. Nevertheless, the increasing geometrical complexity and technical requirements of light metal pressure castings have rendered process stability and castability a permanent issue for foundry men. The castings should be simultaneously light, strong, ductile, weldable, heat treatable, and pressure tight. In order to achieve these goals a process chain approach comprising a significant materials science component must be applied to aid product, alloy and process improvement. The contents of this book involve just such a process chain approach. In addition to the principles of pressure casting, the effects of melt quality, alloy composition, filling conditions, and even post-processing aspects such as heat treatment are addressed. Parameters affecting process stability and productivity are also discussed. It is hoped that this book will help foundry management and foundry men on the shop floor to achieve future improvements in pressure casting.

• Sprache: Deutsch
• Herausgeber: Schiele & Schön
• Erscheinungsdatum: 3. Dez. 2019
• ISBN: 9783794908813

Helmut Kaufmann

Priv.-Doz. Dr. Helmut Kaufmann is Managing Director of the ARC Light Metals Competence Center Ranshofen (LKR) in Austria, and a lecturer in light metals technologies at RWTH Aachen (Germany) and ETH Zurich (Switzerland). Prof. Dr. Peter J. Uggowitzer was appointed Ad Hominem Professor at the Department of Materials of ETH Zurich in 1996. Since 2002 he has been a member of the Laboratory of Metal Physics and Technology, where he is in charge of the Light Metals field.

Buchvorschau

2005

Executive Summary

High-quality casting is a successful synthesis of material science and applied casting technology, which is itself a combination of knowhow from mechanical engineering, process engineering, applied physics and chemistry.

This book is an attempt at a complete overview of light metal alloy pressure casting technology, intended to provide the reader with an insight into this wide field, which is sufficiently deep to at least understand its complexity. For this reason, the text is a deliberate mixture of the scientific and the technical: both a manual for engineers in their practical daily work, and a textbook for students carrying out degree work in materials science and technology.

This volume about modern pressure casting technology rests on the strong foundations of scientific literature and practical experience. The literature on the subject is vast, since strictly speaking the materials science of light metals and foundry process technology should be considered in its entirety. Legitimately, papers on lubricant chemistry and vacuum technology, furnace and heat treatment technology and many other scientific and technological fields of interest should also be taken into account. A mountain of papers have been published, and every day new publications on the subject or related areas appear somewhere on the planet.

The authors wish to indicate from the outset that this work does not attempt a full review or discussion of this literature, but of the large research area of pressure casting. Important publications are nevertheless cited where they were essential for the development of a technology or a fundamental understanding of processes, in order to see the results in perspective to our own work and practical experience.

The authors hope that no important papers have been excluded and apologise to all authors performing valuable research who received no mention in this compilation of papers. Naturally, researchers tend to prefer the latest, the hottest results in the field. Therefore the literature of the past five to ten years has received the most attention, and been discussed the most here.

The High Pressure Die Casting process (HPDC) had its 100th birthday in the year 2005. Significant progress was made during this period in this very complex field of expertise. Even though much has been written about pressure casting in the past, there is still room for a book which takes a comprehensive look at the combination of mechanical engineering and materials science in pressure casting. The authors experienced great personal pleasure when studying the famous textbooks Castings and Castings Practice by John Campbell [Cam03, Cam04]. These two books focus mainly on melt quality and gravity die casting, but are also essential literature for any foundry man. These two books inspired the authors to do something similar for HPDC, which lacks profound textbook attention.

The main focus of well-known existing books is generally the equipment of pressure casting, rather than the metallurgical aspects. At another extreme are textbooks in Materials Science and Physical Metallurgy, which are very theoretical and are probably not referred to by foundry engineers in their daily business.

In the late 1980s and during the 1990s the main focus of HPDC development was on pressure casting machines. Sophisticated new equipment such as HPDC machines with hydraulic shot control, vertical squeeze casting machines and various semi-solid casting machines were developed. The authors tend to call this period the decade of mechanical engineering in pressure casting. In the same period the geometrical complexity of pressure castings and the technical requirements became more severe than ever. Parts became larger and thinner, and needed to be strong, ductile, weldable, heat-treatable, pressure-tight, and inexpensive. Confronted with these requirements, foundry men have realised that despite the modern equipment available, the limits of castability are often exceeded.

This is where pure mechanical engineering comes up against a wall. To solve the problems of the modern foundry business, a process chain approach with specific attention to materials science must replace it. It is this approach upon which this book is based. It addresses not only the principles of pressure casting, but also the effects of melt quality, alloy composition, filling conditions, and post-processing aspects such as heat treatment.

We hope that the reader will enjoy the story, and profit as much as the authors did from the Campbell books.

1 Introduction

In 1905, more than 100 years ago, High Pressure Die Casting (HPDC) came into existence when the first die casting machine by that name was patented in the USA by H.H. Doehler. Two years later E.B. Wagner came out with a prototype of the now familiar hot chamber die casting machine [Sim97]. The first significant use of HPDC was in the production of gas mask parts during World War I. Originally only zinc was used in HPDC, but by 1915 large quantities of aluminium HPDC were already being produced [Vin03].

One hundred years later, HPDC of light metals has become a fully accepted process in the metal casting industry and is applied in basically all areas of mass production. One important user of this technology is certainly the automotive industry.

After a complete century of development in the field of HPDC one question must be asked: is there anything left to be done in research and development of HPDC process and materials? The answer is a clear YES. This book aims to clarify this, scan the current state of development and provide ideas for further research work.

There are two significant drivers of development: quality issues, and cost issues. Both can only be addressed successfully if improvements in the overall process chain and their interactions are considered. Whereas during the 1980s and early 1990s much effort went into the development of improved HPDC machines, the authors note that today more attention is paid to improving alloys, controlling metal quality and optimizing post processing (such as heat treatment).

All measures for product and process optimization must be considered from the quality and cost perspective. In this context it seems appropriate to look at some of the issues discussed in detail throughout this work.

1.1 Quality

HPDC was known for rather porous castings with low ductility and strength levels. Quality improvements were achieved via new processing or machine concepts in the production of high-integrity castings. Vacuum HPDC, Squeeze Casting and the wide field of Semi-solid Casting resulted.

While high speed casting combined with high solidification pressures were initially considered HPDC’s big advantage, it soon became evident that these turbulent, spray-like filling conditions were also the major source of casting defects. Supported by computer simulation, the die filling and solidification processes were optimized, generating improved die designs. Improvements included the gating system, air venting, temperature control, local pressurization and more. Proper die lubrication became an issue in terms of lubrication efficiency when alloys with low iron content were used, and because of their reactivity with the melt (production of reaction gas).

Only very recently have metal quality and alloy composition received proper attention in foundries. It was long generally assumed that HPDC did not produce high-integrity castings with significant ductility, and that therefore metal cleanliness was not really an issue. Cheap secondary alloys (such as AlSi9Cu3) with high iron content in the range of 1% were used. It was not possible to make safety-critical components with this system.

In the past 15 years, however, the complexity of HPDC castings has increased dramatically. Functional integration (actually a cost issue) has forced foundry engineers into new areas of application and demanded new levels of HPDC quality. Thus alloy development for HPDC, melt cleaning, grain refinement, modification, heat treatment and welding have become issues. Functional integration has also enhanced the development of tailored materials and components. Metal matrix composites (MMC) and multi-material compounds (e. g., foam inserts in castings) are some of the results.

1.2 Cost

Modern HPDC equipment is very elaborate, and therefore cost-intensive. To compensate for high investment costs, high productivity (high up-time, low cycle time, high automation grade) is of great concern. Functionally-integrated components usually require complex and mainly large dies, which are expensive. This is why the lifetime of tooling is a major issue. It is closely related to the efficiency of the die lubricant: this should protect the die, but its use should be limited for cost and environmental reasons.

Improvements in post processing (improved heat treatment, reduced quality control, less machining, improved recycling (higher yield rate)) are also required to keep the HPDC process competitive.

Generally it is necessary to develop a very stable process. The part requirements define this process, and the alloy needs. The complexity of the castings and consequently the complexity of the total technology have increased substantially over the past few years. This is a problem in the daily foundry business, because it makes it difficult to run processes with untrained staff. From a cost point of view, however, it would be simply impossible to have a group of Ph.D.s doing this.

Foundry researchers must therefore develop and establish rules for process-oriented alloy development, in combination with materials-oriented process parameters, to reach the required component quality at competitive cost. In-line quality control and immediate countermeasures for any deviation are needed to ensure continuous, highly productive automatic manufacturing of High Pressure Die Castings.

In this work the authors address the optimization of the overall processing chain in terms of alloys, process and quality control, and propose future directions of research in HPDC and related areas.

2 Component requirements for HPDC parts

Modern light-weight design in the automotive industry uses light metal castings with increasing functionality: fewer parts integrate more functions. Therefore the demands on High-Pressure Die Castings have increased steadily over the past few years. Modern HPD castings need to be strong and ductile, and heat-treatable to adjust the mechanical properties to the component needs. In automotive applications this is especially true for suspension parts. Frequently castings need to be large, thin-walled and weldable, as required in space frame nodes or door frames. But sometimes HPD castings also need to be thick-walled and pressure-tight for hydraulic or pneumatic applications. Elevated temperature properties, wear resistance and pressure tightness are important in engine applications. More than previously, HPD castings must possess a special surface quality to fulfil optical requirements or for later adhesive bonding: this is especially true for interior applications such as dashboards. Cast metal surfaces are often merely painted, or wooden, polymer or metallic layers are glued to the cast surface. Roughness and wettability are important for the latter use.

Strong, ductile and pressure-tight parts require proper alloys and castings with limited porosity and inclusions. For heat-treatable castings gas inclusions resulting from die filling, hydrogen reactions from wet foundry equipment and lubricants or carbon reactions from lubricants must be kept to a minimum. The same is true for the weldability of HPD castings. The choice of lubrication is also critical when it comes to surface quality. If large, thin-walled castings need to be manufactured cost-efficiently, solutionizing heat treatment should be avoided to reduce energy consumption for heat treatment and the rectifying operation to eliminate distortion of the castings after quenching. New naturally-ageing alloys with high strength and ductility or T5 heat treatment of existing or slightly modified alloys may be the solution.

In principle all of these requirements apply both to castings made of aluminium, and magnesium alloys. Alloy composition, melt treatment, melt handling and ladling into the machine have a significant influence on subsequent casting quality. Later die filling can only cause the quality of the melt to deteriorate, and not improve it.

The preparation of the melt, its ladling into the high pressure die casting machine and the die filling process are the parts of the processing chain where the various existing HPDC processes differ the most. High solidification pressure, however, is a property common to all HPDC processes. It is applied to enhance feeding during solidification of the melt in order to avoid shrinkage porosity. The various HPDC processes resulted from the application (and sometimes varying interpretation) of modern casting theory. Theoretical considerations concerning die filling will be provided in Chapters 3 and 4, followed by an introduction to the modern HPDC processes which resulted (Chapter 5).

3 About the effect of pressure on the solidifying melt

Overcoming the shrinkage porosity caused by contraction of an alloy upon solidification has always been a major challenge for foundry men wishing to produce high-quality castings. According to [Gho00], Chernov was the first to suggest, in 1878, the application of external steam pressure on solidifying molten metal to enhance feeding.

The feeding of the remaining liquid is improved by increasing the pressure on the melt. The feeding velocity is controlled by Darcy`s law, which states that it is a function of the applied pressure gradient and the feeding resistance, which depends on the permeability of the already-solidified phases.

v0 = -K/μ · ΔP          [3-1]

In this equation K is the permeability, m the viscosity of the melt (e.g. eutectic liquid) and DP the pressure drop.

It is known that permeability is a function of solid fraction. Feeding stops suddenly when the permeability is too high and the pressure difference is insufficient to force metal into the interdendritic space. The process of feeding, similar to the infiltration of fiber preforms by molten light metals, can be described relatively easily for the isothermal case of gas pressure infiltration [Kauf92, Kauf93], but becomes very complicated when the permeability is a function of time due to solidification.

As just stated, solidification time and therefore the available feeding time limits the success of feeding due to externally-applied pressure. In thin-walled High Pressure Die Casting the external pressure cannot support feeding as much as it can in thick-walled squeeze casting, where runners and gating systems are designed for improved feeding. However, applied pressure not only helps reduce shrinkage porosity due to improved feeding: it may also alter the microstructure due to better heat transfer into the surrounding die material, and due to a possible grain refinement effect following extensive undercooling of the melt. This effect can be attributed to a change in the phase diagram following the Clausius-Clapeyron equation

ΔTf/ΔP = Tf(V1-Vs)/ΔHf         [3-2]

where Tf is the equilibrium freezing temperature, Vl and Vs are the specific volumes of the liquid and the solid respectively, and DHf is the latent heat of fusion [Gho00]. Substituting the appropriate thermodynamic equation for volume (by taking the liquid metals as an ideal gas), the effect of pressure on the freezing point may be estimated roughly as follows:

P = P0exp(-ΔHf/RTf)         [3-3]

P0, ΔHf and R are constant, and Tf increases with increasing pressure during solidification. Equation 3-3 leads to a change of the liquidus and solidus lines in a binary phase diagram, as shown in Figure 3-1.

Figure 3-1: Change of liquidus and solidus lines in the binary Al-Si phase diagram following rapid solidification under high pressure [Gho00].

During the filling phase the melt experiences no, or practically no, external pressure above atmospheric pressure, but it may still drop in temperature close to or even just below liquidus. Immediately after die filling is completed, external pressure is applied. The latter can reach up to 150 MPa in today’s squeeze casting machines. In High Pressure Die Casting machines roughly 100 MPa is the upper limit. For binary Al-Si alloys an increase in the liquidus temperature by about 9°C was shown when solidification took place under 150 MPa [Gho00].

With this external pressure on the melt – which may be slightly undercooled at the moment of pressurization – at a given temperature, the phase diagram changes according to equations 3-2 and 3-3 and the melt is suddenly in a much more undercooled state than moments previously. This thermal undercooling stimulates existing nuclei in the melt to commence spontaneous heterogeneous solidification, resulting in much finer grain structures than in the same melt solidified under atmospheric pressure. Yong and Clegg observed a reduction in cell size of the magnesium alloy RZ5DF (4.2% Zn, 1 % Rare Earth) from 127 mm for gravity and 21 mm for direct squeeze casting [Yon04], leading to an improvement in strength of about 25%.

In this context the question of additional grain refinement and modification of light metal alloys must be discussed. Here it is difficult to elaborate a definite rule. The geometry of the casting and especially the wall thickness and solidification time must be taken into consideration. The heat transfer coefficient changes when internal metallostatic pressure from the feeding system suppresses the gap formation during the solidification of thin-walled castings [Cam03], and therefore external pressure alone will generate an improvement in the microstructure. However, further improvements may also result from altering the alloys. This subject is dealt with in Section 7.3, which covers grain refinement and modification.

From the microstructural point of view there is also one negative aspect, often attributed to the application of high external pressures on solidifying melts: macrosegregation. There seem to be two types, distinguishable according to the time when the separation of phases occurs. Primary phase and eutectic can separate during filling if pre-solidified primary phase is present, or during feeding under pressure, when a relative movement of solid and liquid phases (due either to shrinkage gap formation or to incomplete filling) is possible.

Recently published research work investigated this phenomenon mostly in the direct Squeeze Casting process [Bri03, Hon00], but it is also a familiar issue in indirect Squeeze Casting and Semi-solid Casting. Macrosegregation after filling may be less of a problem in conventional high pressure casting of thin-walled parts, but phase separation during filling is very prevalent.

St.John et al. report an increased volume fraction of eutectic Mg17Al12 at the skin layer on Mg-HPDC parts and attribute this to normal fluid flow phenomena during flow of semi-solid slurries (assuming that pre-solidified particles have been transferred from the sleeve into the die) [StJ03]. The formation of such a layer can significantly alter the corrosion behaviour of a part in machined or as-cast areas [Küh02].

In the authors´ opinion the basic requirement for the formation of macrosegregation after filling of the die is the potential for relative movement of solid and liquid phase. Sufficient amounts of liquid and sufficient time for movement are also essential. While the former may be present in thin-walled HPDC parts, there the solidification time is so short and wall-thickness so thin that the permeability of the solid is too high for relative movement during the very short time after pressurization. In Squeeze Casting and Semi-solid Casting the situation is quite different, since the cross-sections of the parts, the runners and the gating system are usually thicker, and solidification takes longer.

A simple example of relative movement of solid and liquid is seen when the solidifying melt begins to contract. Gap formation between the die and a first, weak solid shell is a phenomenon familiar to all foundry workers. Associated with a drop of heat transfer, it can generate huge gaps in large castings. Campbell [Cam03] describes cases of large castings of about 1 meter in length where gaps of 10mm on opposing sides of the die can develop. There is, however, a significant pressure drop between the gap area, which is under atmospheric pressure (it pulls air through ejector and venting channels), and the melt, which is subject to sufficient external pressure. This pressure drop can enable the liquid to flow back into the gap. It does not necessarily mean that the applied pressure ruptures the solid shell, which is in any case under tensile stress upon contraction. The thin shell may even remelt locally due to the temperature increase after the drop of heat transfer coefficient at first contraction.

It is mentioned above that extensive undercooling causes solidification to start throughout the melt once high pressure is applied. This means that primary solid crystals and eutectic liquid also coexist in the pool of melt inside the solidified shell. The permeability of the already-solidified layer may be so low, and the available channels so narrow that only the eutectic liquid can flow into the gap, while the primary crystals are basically filtered by the solidified shell. This type of macrosegregation is shown in Figure 3-2. A casting was made by New Rheocasting (NRC) with the alloy AlSi7Mg at about 50% solid fraction. The die filling of an end section was incomplete prior to pressure intensification. Upon pressurization the oxide shell ruptured, the solid phase was immobile and the remaining liquid filled the gap.

Figure 3-2: Macrosegregation in a New Rheocasting section of the alloy AlSi7Mg at roughly 50% solid fraction due to gap formation between the die and the part prior to pressurisation. The oxide skin ruptured and mainly liquid fraction moved into the available space.

This can only be avoided if large relative movements of liquid and solid phases can be kept to a minimum. This requires complete die filling (sufficient wall thickness, large radii) and low levels of gas entrapment. Collapsing gas bubbles can also be a source of macrosegregation, as shown in Figure 3-3.

Hong et al. [Hon00] observed that macrosegregation in a direct squeeze cast AlSi7 alloy was reduced significantly when heterogeneous grain refinement via TiB2 inoculation was applied. This observation agrees well with the theory of the formation of macrosegregation in pressure casting. As shown by Dahle and St.John [Dah99], the coherency of the solid fraction is delayed when grain refinement is applied to the alloy. This results in delayed contraction of the solid and, therefore, in delayed gap formation between the die and the contracting alloy. Since this gap forms later, when more solid is present, permeability is higher and even external pressure may not be able to force eutectic liquid into the gap.

Figure 3-3: Flow of eutectic liquid into the area of a collapsing gas bubble, passing the permeable solid network [Cam03].

Dahle and St.John discussed dendrite coherency in their work on the formation of band defects in pressure castings. Upon solidification an alloy goes through three stages of strength/structure relationship. For temperatures below dendrite coherency, the already solidified phase(s) can move freely in the melt and no strength can be attributed to the slurry. Between the coherency point and the point of dense packing, strength develops slowly. At temperatures below the temperature for dense packing, strength increases quickly. All three stages may be present in a die at one given moment, and all three types of slurry or solid react differently to applied shear.

Banded defects are more likely to occur in high pressure cast magnesium than aluminium alloys since, for example, AZ91 has a solidification interval more than five times larger (170 °C compared to 30 °C) and only half the amount of eutectic than the aluminium alloy AlSi7Mg (25 % compared to 50 %) [StJ03].

Dahle and St. John summarized a few suggestions for the control of banded defects in pressure castings: reduce the number and/or size of overflows; reduce shot sleeve solidification; increase die temperature; increase melt temperature; and ensure that pressure is only applied after the die is completely filled [Dah99].

Finding the proper casting conditions for any casting always results in the search for a set of parameters, which is the best achievable compromise. While high metal temperature may help to limit banded defects, it may also enhance die soldering and hydrogen pick-up, cause trouble with cover gas in magnesium casting, and more. A reduction of overflows may reduce banded defects in the area where the partially solidified metal is extruded into the overflow region, but it may also generate entrapped gases and oxides in this area.

We will examine these issues in more detail throughout this work, but can already state that a definite set of rules for setting up perfect pressure casting is impossible to elaborate. The influences of part geometry, process and alloy are simply too strong. Recent findings on macrosegregation in semi-solid casting are reported in Section 9.3.

4 Turbulent and laminar flow

One requirement dominates the production of reproduceable castings: the die cavity must be completely filled. Until the late 1980s HPDC was mostly considered a casting process for thin-walled housing components or for castings where pressure tightness and ductility were of no real concern. During this early period in HPDC the leading credos for successful production were high speed and high end pressure during solidification. Process parameters still considered very important in HPDC are ingate speed, plunger speed and filling time. To secure complete filling and liquid flow to the top end of thin-walled castings, thin ingates and gate speeds higher than 50 m/s were applied, and are unfortunately still often used, in conventional HPDC.

The past 20 years in HPDC R&D have been dedicated to resolving the drawbacks of fast and turbulent filling. Campbell [Cam03] dedicated almost a whole book to describing the negative effects of oxide films entrapped in castings. Excessive die filling speed is considered a major source of trouble.

4.1 Critical ingate speed

Based on an energy balance between a surface tension term and the kinetic energy of the flowing melt, the critical ingate speed where a stable metal front would turn into an advancing metal finger or spray was derived [Cam03]. The critical ingate speed vcrit can be calculated according to Equation 4-1:

vcrit = 2(γsg/ρ)¼         [4-1]

Here γs is the surface tension of the melt and ρ its density. g describes the acceleration due to gravity.

Most important engineering metals have critical speeds close to 0.5 m/s. This means that any metal front leaving a thin film ingate and advancing into the wider cavity of a HPDC die at gate speeds of 50 m/s would spray into the die instead of flowing. The melt droplets are immediately covered with thin oxide layers and then compacted after completed filling during solidification. These droplets form a network of brittle oxides with compressed gases in between. Under such filling conditions even normally ductile alloys show only low elongation values in tensile testing.

The focus on ingate speed assumes that the ingate is the narrowest area of the casting. In modern complex castings with changes in wall thickness throughout the component such ingates may be located in various places in the part and must be carefully considered during parameter evaluation for the shot profile.

To avoid fingering or spraying of the melt during die filling, there are only two options: reduce the flow speed or increase the surface tension. The first would lower the ingate speed below the 0.5 m/s mentioned above, while the latter would increase the critical speed to a higher level. Increased surface tension, gs, is certainly functional in semi-solid casting of light metals, but to the authors’ knowledge exact data for gs are unavailable. This also makes it impossible at present to determine the Weber number for die-filling during semi-solid casting.

4.2 Weber number We

The concept of critical velocity for entrainment of the surface and surface turbulence is enshrined in the dimensionless Weber number, We [Cam03].

We = ρLv² / γs         [4-2]

In Equation 4-2 γs is the surface tension of the melt and ρ its density. L describes a characteristic length (i.e. the hydraulic radius), and v the flow speed. Weber numbers can be calculated for fully liquid flow, and can be helpful in determining the process parameters. It is expected that We <1 represents flow conditions without surface turbulence.

In order to measure the surface tension of semi-solid liquids, further basic research is required. The most critical point is certainly the shear-rate-dependent behaviour of melts in the semi-solid state.

4.3 Reynold`s number Re

The most commonly-applied dimensionless number for characterizing flow behaviour is the Reynold`s number:

Re = ρvL / μ         [4-3]

In Equation 4-3 ρ is the melt density, and m its viscosity. v and L again describe flow speed and characteristic length. While the Weber number is meant to give indications of the surface condition, Re describes the state of the bulk flow. If Reynold`s numbers are smaller than 2300 the bulk flow is laminar. Practical experience has shown that in real casting operations with fully liquid melts it is very difficult to achieve laminar flow conditions and still reach complete die filling.

Equations 4-1 to 4-3, describing the critical ingate speed vcrit, surface turbulence and bulk turbulence, indicate that three parameters govern flow behaviour in real casting: flow speed v, surface tension γs, and viscosity m. In the search for proper casting parameters in the context of local flow speeds it is usually assumed that the flow channels are fully open. In reality this can be quite wrong, since solid shells may form quickly in pressure casting zones where unidirectional flow is required. These shells can drastically reduce the effective flow area and thereby generate turbulent flow, as the increase in local flow speed and the Reynold`s number would indicate. For a circular solid shell of 1 mm in a runner of 10 mm in diameter the flow speed would increase by more than 50%, given that the volume flow remains constant! At given shot conditions and similar viscosity levels the difference in density can lead to laminar flow for magnesium alloys, while aluminium alloys would already flow in a turbulent manner.

In the development of modern HPDC machines and new processes, foundry engineers have tried different approaches to utilize this theory. Vertical and Horizontal Squeeze Casting (and derivatives such as Poral